Tidal deposits with climbing ripples in the Pleistocene Shimosa Group

Abstract

The Izumiyatsu Mud is included in the lower part of the Jizodo Formation of the Pleistocene Shimosa Group. A lenticular fine-grained sand bed with climbing ripples is intercalated in the mud member. The sand bed can be divided into two units by a mud drape, and each bed shows opposit paleocurrents, ebb and flood directions. Moreover, time variation of the types of climbing ripple laminae within the sand bed also indicates a systematic variation of stream energy (velocity and/or sediment supply), which shows the same variation of that of tidal current. Thinly bedded alternation of sand and mud layers, about 70cm thick, overlies the climbing ripple sand bed. The thickness of the sand layers in the alternation changes cyclic pattern which can be explained by neap-spring tidal cycles.